I've had a Gsync monitor since October of last year, 27" Asus 144hz 1440p, and been happy with it, and another since January, an Acer 4k 60hz, which I've been so-so on. The Gsync works well with both, and does make a big difference in many games, particularly first person shooters.
To enable it, it's in the nVidia control panel, they've changed where it is an how it works a couple times in the last year, but it's there if you have the hardware and the latest drivers installed.
It's getting cheaper, and more monitors are coming out with one or the other. If you favor nVidia like 3/4 of the gaming public does (according to various stats I've seen), at least for now, Gsync is the way to go IMO. It seems that Freesync has a higher ratio of supporters, probably due to the cost, as stats say it's about even with use in terms of comparing it to Gsync, yet there are many, many more nVidia users overall. I've only seen Freesync, not tried it, but if you have a newer AMD card I'd certainly look into it, Asus has a Freesync 144hz IPS monitor that's around 500$ now, something I would seriously consider with a good AMD card.
144hz is the most important factor in a gaming monitor still, more so than G or Freesync IMO. If you can only afford so much, stretch for a 144hz monitor, preferably an IPS one, but the TN 144hz Asus I've owned (3 now) have all been excellent. I'm going to get 3 of the new Acer 144hz Gsync IPS as they look to be the best bang/$ out there now, but I'm happy with the current Gsync stuff now after a year as well. The XB270HU is probably the best gaming monitor out there now, much the same as my current Rog Swift Asus, but IPS instead of TN, which makes a big difference IMO.
